The disclosure relates to lubricating compositions that provide good oxidative stability and reduced sludge and varnish deposits. The compositions are particularly suited for power generation devices, such as gas, steam and combined cycle turbines, as well as in other industrial fluids such as industrial gear oils, hydraulic fluids, and other circulating oils.
A turbine is a device used to generate electricity or mechanical power through rotational movement of a shaft. Gas and steam turbines use a flow of hot combustion gas or steam to generate energy in the form of thrust and/or shaft power, in any combination. For example, air flowing into the gas turbine is compressed in an air compressor and fed, at high temperature and pressure, into the combustion chamber where fuel is injected and the resulting fuel/compressed air mixture ignites. The rapidly expanding gases resulting from the ignition exit the combustion chamber at high velocity, pass over the turbine blades, and thus cause the turbine shaft to rotate. Steam and combined cycle units operate in a similar manner.
Gas, steam and combined cycle power generation units are often operated in extreme environments and exposed to changes in atmospheric pressure, changes in ambient temperature, water, sea water, dust, and a host of other liquid and solid contaminants. Sludge and other deposits are particularly undesirable in power generation units used in a peak-load or cyclic manner. In such circumstances, the turbine will be activated and put into service for relatively short periods of time to meet peak loads on the electrical grid. Once the demand softens, the units are shut down and the oil stops circulating. Sludge and other deposits are more likely to settle out of the oil composition as the oil cools down to ambient temperature. The problem is aggravated by repetition of this heating-cooling process and also probably the stagnation of the oil. Use of Group II base stocks, which have become popular in recent years, has been linked in some cases to increased sludge and varnish deposit formation. Therefore it is of benefit to reduce the formation of sludge and other deposits in turbine fluids and thus reduce the need for expensive turbine maintenance and financially detrimental system downtime. Similar concerns are present in industrial gear applications, hydraulic fluids and in other circulating oils. A number of tests are known to determine the oxidative stability of lubricating compositions. The most common are ASTM D2272—Rotary Pressure Vessel Oxidation Test (“RPVOT”) and ASTM D943—Turbine Oil Stability Test (“TOST”). The fact that a particular antioxidant package performs well in these oxidative screening test, however, does not necessarily guarantee that it will be effective to control sludge and other deposits. A more stringent test is the “MHI Dry-TOST” as disclosed in Mitsubishi Heavy Industries MS04-MA-CL002, MS04-MA-CL003 and MS04-MA-CL005 (draft) specifications. This test measures both the resistance of an oil composition to oxidation and also the potential for deposit formation in the composition.
Thus, there is a need for lubricant compositions having excellent oxidative stability and minimal deposit and sludge formation.